The present invention relates to a controllable damping force hydraulic shock absorber which is mounted on a suspension apparatus of a vehicle such as an automobile.
As a hydraulic shock absorber mounted on a vehicle such as an automobile, there is known a controllable damping force hydraulic shock absorber in which damping force characteristics can be appropriately controlled according to road surface conditions, vehicle running conditions, etc., so as to improve ride comfort and steering stability.
Generally, a controllable damping force hydraulic shock absorber is arranged as follows. A piston, which has a piston rod connected thereto to form a piston assembly, is slidably fitted into a cylinder in which a hydraulic fluid is sealably contained. The piston divides the inside of the cylinder into two chambers. The piston assembly is provided with a main hydraulic fluid passage and a bypass passage which permit communication between the two chambers in the cylinder. The main hydraulic fluid passage is provided with a damping force generating mechanism including an orifice and a disk valve, and the bypass passage is provided with a damping force control valve for changing a flow path area of the bypass passage.
In this arrangement, a small damping force is generated by opening the bypass passage through the damping force control valve so as to reduce a resistance to the flow of the hydraulic fluid between the two chambers in the cylinder. On the other hand, a large damping force is generated by closing the bypass passage so as to increase the flow resistance between the two chambers. Thus, damping force characteristics are controlled by opening/closing the bypass passage through the damping force control valve.
However, when a damping force is controlled only by changing the flow path area of the bypass passage, the following problem arises. That is, although damping force characteristics can be changed to a large extent in a low piston speed region in which a damping force is dependent on the restriction of an orifice in a fluid passage, damping force characteristics cannot be greatly changed in intermediate and high piston speed regions in which a damping force is dependent on the degree of opening of the damping force generating mechanism (disk valve) in the main hydraulic fluid passage.
As a countermeasure, there is known a controllable damping force hydraulic shock absorber, as disclosed in Unexamined Japanese Patent Application Public Disclosure (Kokai) No. 7-332425 (corresponding to U.S. Pat. No. 5,655,633), in which a pilot type damping force control valve is provided as a damping force generating mechanism in each of a main hydraulic fluid passage for an extension stroke and a main hydraulic fluid passage for a compression stroke. In this pilot type damping force control valve, a back-pressure chamber (a pilot chamber) is formed at the back of a disk valve. The back-pressure chamber is communicated through a fixed orifice with a cylinder chamber on the upstream side of the disk valve and communicated with a cylinder chamber on the downstream side of the disk valve through a flow rate control valve (a pilot control valve).
In this controllable damping force hydraulic shock absorber, the flow rate control valve is operated, to thereby change the flow path area of a flow passage between the two chambers in the cylinder, while the pressure in the pilot chamber is changed due to a pressure loss at the flow rate control valve, to thereby change the valve-opening pressure of the disk valve. Thus, orifice characteristics (in which a damping force is approximately proportional to the square of the piston speed) and valve characteristics (in which a damping force is approximately proportional to the piston speed) can be controlled simultaneously, thus making it possible to control damping force characteristics within a wide range.
The present assignee filed, in Japan, Japanese Patent Application No. 2001-164748, with respect to a controllable damping force hydraulic shock absorber in which a pilot type damping force control valve is provided in each of an extension-stroke passage for allowing flow of a hydraulic fluid during an extension stroke of a piston rod, and a compression-stroke passage for allowing flow of the hydraulic fluid during a compression stroke of the piston rod; a damping force for an extension stroke and a damping force for a compression stroke are, respectively, controlled by means of the pilot type damping force control valve in the extension-stroke passage and the pilot type damping force control valve in the compression-stroke passage.
However, in the above-mentioned conventional controllable damping force hydraulic shook absorbers in which a pilot type damping force control valve is provided in each of the extension-stroke passage and the compression-stroke passage, the following problems arise. A pilot chamber of the pilot type damping force control valve provided in the extension-stroke passage is pressurized during an extension stroke and depressurized during a compression stroke. On the other hand, a pilot chamber of the pilot type damping force control valve provided in the compression-stroke passage is pressurized during a compression stroke and depressurized during an extension stroke. Thus, when the direction of stroke of the piston rod is reversed, the pilot chambers are subject to a considerable change in hydraulic pressure. As a result, valve bodies and seal members move and collide against valve seats and seal portions. This causes vibration and pronounced noise. Further, there is a possibility of the valve bodies and seal members being damaged due to chattering. Further, the time required for pressurizing the pilot chamber which has been in a depressurized condition is relatively long, so that a first order lag in development of a damping force occurs, thus making it difficult to obtain a desired damping force.
Further, when a pressure control valve capable of directly controlling a hydraulic pressure is used for controlling a pressure in the pilot chamber, the pressure control valve is likely to cause chattering, which results in an increase or decrease in the pressure in the pilot chamber. Therefore, a stable damping force cannot be generated.